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  <head>
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    <title>Document</title>
    <style>
      .box {
        padding: 40px;
        background: #fcf0f0;
        position: fixed;
      }
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    <style>
      /* 在这里完成作业 */
    </style>
  </head>
  <body>
    <div class="box">
      <p>这是一个固定定位的盒子</p>
      <p>它的宽高是不固定的</p>
      <p>请将它居中</p>
    </div>
  </body>
</html>

